In recent years, signal transmission systems utilizing optical fibers as the transmission medium have gained increasing popularity due to their wide bandwidth, small size and relative immunity to electrical noise. Such systems include a laser diode or other light source for producing a light beam, means for modulating the light beam, an optical fiber for guiding the light beam between the source and a destination, and a photodetector at the destination. A plurality of modulated channels can be carried on the optical fiber simultaneously to increase its information-carrying capability. The channels can be demultiplexed at a single location or at a number of different locations. For example, a telecommunications system requires taps to a main trunk line at different subscriber locations. Each subscriber may utilize one of the channels being carried on the main trunk. Accordingly, there is a need for devices to remove a selected channel or band of channels from an optical communication line with minimal disturbance to the other channels carried thereon.
Prior art wavelength division multiplexing systems include a multi-transmitter module at one end and a multi-receiver module at the other end. The systems are contemplated for trunks and are not easily adaptable for systems tapped at arbitrary places without first detecting all the signals and then regenerating them. Multiplexing and demultiplexing means typically comprise gratings, prisms or filters. While these devices are adequate for many multiplexing systems, they suffer the drawback that the number of channels that the system may handle is limited because the wavelength dispersion of the demultiplexing device is not adequate to separate very closely-spaced channels in devices of reasonable dimensions. They suffer the additional disadvantage that all channels (or frequencies) are removed at once. They cannot be used conveniently to remove one or a few channels and allow the remainder to continue along the trunk. Moreover, such devices suffer the disadvantage that only a fraction of the channels can be captured in branch fibers due to the small ratio of core diameter to cladding diameter in commercial fibers. When a fix tuned tap is to be used without feedback or continual tuning, it must remove a broad enough portion of the spectrum so that changes in its tuning due, for example, to temperature variations and aging will not shift the removal spectrum or its width by more than a small fraction of the removal spectrum width. Also, some of the multiplexing devices have a relatively high fixed loss per channel. This also limits the possible number of channels.
Various devices and techniques useful in constructing optical couplers and optical filters have been disclosed in the prior art. Lateral coupling structures utilizing optical fibers are disclosed in U.S. Pat. No. 4,315,666 issued Feb. 16, 1982 to Hicks, Jr. Various structures for obtaining close coupling between fiber cores are described. Such lateral couplers work on the principle of coupling of the evanescent waves on two closely-spaced optical cores. The energy on one core gradually is coupled to the other core. Lateral coupling of optical waveguide energy to resonant cavity filters is disclosed in International Publication No. W083/02168, Hicks, Jr., published June 23, 1983 and International Publication No. W0 85/00484, Williams, published Jan. 31, 1985. The cavity resonators disclosed in these publications are analogous to Fabry-Perot interferometers and comprise a length of optical fiber having mirrors at each end. The length of the optical fiber is chosen to be an integral number of half wavelengths at the selected resonant frequency. The resonator can be linear or in the form of a loop. In the prior art, lateral coupling has been utilized for coupling energy into optical resonant cavity devices. This arrangement has the disadvantage that, to obtain wide bandwidth resonant cavities, for example 10.sup.10 Hertz, lengths on the order of 100 micrometers are required. However, lateral coupling devices require lengths on the order of one centimeter. Thus, lateral coupling to wide band filters has not been possible.
It is an object of the present invention to provide an improved filter tap for optical communication systems.
It is another object of the present invention to provide a filter tap for optical communication systems wherein the main trunk line is optically coupled to the end face of an optical resonant cavity.
It is yet another object of the present invention to provide a filter tap for optical communication systems with minimal disturbance of unselected channels.